1. Field of the Invention
This invention relates to an improvement in a method and apparatus for indirectly heating and/or conveying carbonaceous solid particles in a dense phase flow through transfer lines. More particularly, this invention relates to an improvement in a process for reacting coal particles in coal conversion processes.
2. Description of the Prior Art
Increasing energy needs have focused attention on solid fossil fuels due to their availability in the United States in a relatively abundant supply and their potential value if converted into more useful forms of energy and feedstock. Processes such as carbonization, hydrocarbonization and hydrogasification, wherein synthetic fuel products have been prepared by introducing a fluidized stream of finely-divided coal particles into a reaction zone and reacting the coal particles at elevated temperatures in the presence of inert gases, air, steam, oxygen, hydrogen, or the like, are well known. Fluidizing gases such as air, nitrogen, steam, hydrogen or the like are usually employed.
It is desirable to preheat the coal particles in many of these processes before the particles are reacted with a suitable reagent at elevated temperature in the reaction zone. In the past, coal particles have been directly preheated by means such as contacting the coal particles with large quantities of hot gases. This approach was used since coal particles were conveyed through transfer lines in dilute phase flow in these processes. By "dilute phase" as employed throughout the specification is meant a concentration of solids in fluidizing gas of from about 1 pound to about 2 pounds of solid per cubic foot of gas. Large quantities of hot gases, however, required that costly large diameter lines be employed throughout to accommodate the gases. A serious disadvantage of conveying and heating coal particles in dilute phase flow was that due to the required high velocity in the line which was in the order of more than 75 feet per second, erosion of the line was significant. This erosion made wear plates mandatory in various sections of the line, particularly at bends. Moreover, since it was undesirable to feed the large quantities of hot gases into the reaction zone along with the coal particles, the coal particles were separated from the inert heating gases by suitable equipment such as a cyclone separator.
Indirect heating of a coal-gas mixture flowing through transfer lines in a dilute phase was uneconomical and impractical due to the inherently poor heat transfer coefficients of the transfer lines in dilute phase flow, approximately 1 BTU to 2 BTU per hour per .degree.F per square foot of inside surface area of the transfer line. As a result, indirect heat transfer required the expenditure of much heating energy. For this reason, in coal conversion processes employing a dilute phase flow of coal particles, pheheating steps have generally involved directly contacting the coal particles with hot metal pipes situated in a preheat zone or with large quantities of hot inert gases.
Albright, Holden, Simons, Schmidt, Chem. Eng., 56 103(1951) reported the dense phase transfer of coal particles employing a pneumatic feeder and U.S. Pat. No. 3,337,417 disclosed heating a stream of gas fluidized coal in a dense phase while passing through a tubular heat exchanger immediately prior to carbonization. However, transfer lines on a large scale having a constant diameter throughout would erode rapidly if a dense phase flow of coal particles were to be employed.